I have been using the S8 exclusively since launch. I had not noticed any lag whatsoever on the launcher. Now that I have been using the brand new OnePlus 5 for the past few days the S8 feels a bit sluggish. Samsung has just rolled out an update to "fix lag" on the Touchwiz launcher. It fixes a specific bug that causes devices to stutter when opening and closing apps. You can grab the update from Google Play, or from Samsung's Galaxy App Market.
via Touchwiz Home